1. Field of the Invention
The present invention relates to an image display apparatus that includes a liquid crystal spatial light modulating element and is configured to conduct time division display of a frame by dividing the frame into plural sub frames. The present invention also relates to a method of driving a spatial light modulating element with an alternative current.
2. Description of the Related Art
In recent years and continuing, with the dramatic increase in computer processing capabilities, technology for displaying a higher resolution image in a computer is developing and in turn, high resolution capabilities are being demanded in an image display apparatus. However, in apparatuses such as a projector, the resolution of a spatial light modulating element for displaying an image may not be able to keep up with the increasing resolution of the image to be displayed. Accordingly, various techniques are proposed for achieving a higher resolution image in such apparatuses. For example, in Japanese Laid-Open Patent Publication No.2003-90992, a configuration of a projector including an optical deflecting element is disclosed.
FIG. 1 shows a configuration of a projector according to the disclosure of Japanese Laid-Open Patent Publication No.2003-90992. The projector illustrated in this drawing includes an LED light source that is formed by a two-dimensional array of LED lamps and is arranged to emit light towards a screen, a diffuser positioned in the light emitting direction with respect to the light source, a condenser lens, a spatial light modulating element in the form of a transmissive LCD panel, and a projection lens. Further, the projector includes an optical deflecting element that is arranged at the light path between the transmissive LCD panel and the projection lens.
In this projector, illuminating light that is emitted from the LED light source is equalized at the diffuser, and is controlled and synchronized with an illuminating light source by the condenser using an LCD panel drive part to realize critical illumination of the illuminating light onto the transmissive LCD panel. Then, the illuminating light is spatially modulated by the transmissive LCD panel and is incident onto the optical deflecting element as image light. The image light is then shifted by a given distance in the positioning direction of pixels.
Also, in this projector a panel drive part is arranged to divide an image of one frame into plural sub frames so that the sub frames may be sequentially displayed through time divisional display at the transmissive LCD panel. In turn, the optical deflecting element deflects the light path in synch with the display of each sub frame so that the display positions of the sub frames may be shifted with respect to each other. In this way, it may appear as though the number of pixels displayed is multiplied.
The optical deflecting element may correspond to a device for deflecting the light path of incident light and may be realized by forming a lateral directional electrical field on a chiral smectic C liquid crystal layer. According to this example, an average inclination of the optical axis of the chiral smectic C liquid crystal layer, which generates its own polarity, is used to change the electrical field direction to enable high speed light path deflection.
When liquid crystal is used to realize the spatial light modulating element, an alternating current needs to be applied as the drive current for the spatial light modulating element as is known among persons skilled in the art. When a direct current is used as the drive current, the liquid crystal may be susceptible to damage such as burns. Accordingly, in a display apparatus such as a projector including an optical deflecting element that divides an image of one frame into plural sub frames and sequentially displays the sub frames through time division display, a frame polarity reversing drive method is conventionally used for switching the polarity of a drive voltage for the liquid crystal spatial light modulating element. Examples of such a method are illustrated in FIGS. 2˜4 and are described in detail below.
FIG. 2 illustrates an example of dividing a frame into four sub frames and displaying the sub frames with the spatial light modulating element through time division display while changing the light path with the optical deflecting element so that one pixel of the spatial light modulating element displays four pixels on the display screen. The arrows shown in this drawing represent the pixel shifting direction of a light path shifting unit.
FIG. 3 shows a case in which the pixels of the spatial light modulating element displays the same image of FIG. 2 on the screen.
FIG. 4 illustrates an exemplary operation of signals for driving the spatial light modulating element. In this drawing, Vsync represents a vertical synchronizing signal that is generated in sync with a display period T2 for one sub frame. Vd represents a drive voltage that is applied to a given pixel electrode of the spatial light modulating element. Vcom represents a voltage that is applied to a common electrode. Generally, each pixel is driven by a difference voltage (Vd−Vcom) corresponding to a difference between the drive voltage Vd that is applied to a corresponding pixel electrode and the voltage Vcom that is applied to an opposing electrode corresponding to a common electrode for the pixels that is arranged on the other side of the liquid crystal layer.
It is noted that periods [1] through [4] indicated in this drawing represent the respective sub frame periods making up one frame. As is indicated in the drawing, the polarity of the drive voltage Vd within one period T1 is reversed with respect to the voltage Vcom in a next frame period T1, and in this way, the pixels of the spatial light modulating element may be driven by an alternating current.
However, when a drive method of reversing the polarity for every frame is used, the alternating current drive frequency may be decreased causing a perceptible flicker on the display screen.
Accordingly, a method of increasing the alternating current drive frequency to prevent such a flicker effect is known. Japanese Laid-Open Patent Publication No.6-27902 discloses one example of such a method in which the polarity of the drive voltage Vd is reversed for every vertical synchronization period, that is, for every sub frame period. However, in the case of applying such a method to a projector having a light path shifting unit (e.g., optical deflecting element) as is described above, the applied drive current may end up being a direct current, and thereby, the method may not be directly applied. For example, FIG. 5 illustrates the drive voltage Vd of the spatial light modulating element in the case of applying this method to the projector described above. It may be easily understood from this drawing that the drive voltage Vd in this example ends up being a direct current rather than an alternative current.
Japanese Laid-Open Patent Publication No.6-38149 discloses another exemplary method for preventing the flicker effect in which the polarity of the voltage Vd is reversed with respect to every one or more horizontal synchronizing periods and every vertical synchronizing period. However, this method also creates the same problem as is described above.
Japanese Laid-Open Patent Publication No.7-175443 discloses a method of converting the drive voltage into an alternating current by reversing the drive voltage polarity with respect to every vertical synchronizing period within one sub frame period, the method being realized in a case where a sub frame period and a vertical synchronizing period do not correspond and at least two vertical synchronizing periods are provided within one sub frame to realize high speed writing of image data. FIG. 6 corresponds to a diagram illustrating such an arrangement.
However, in a display apparatus that is arranged to temporarily display a predetermined gray level (e.g., black display) at the spatial light modulating element upon switching the sub frame display, unless the drive voltage for temporarily displaying the predetermined gray level with the pixels of the spatial light modulating element upon switching the sub frame display corresponds to zero, asymmetry is created between positive and negative voltages so that the applied voltage corresponds to a direct current. FIG. 7 illustrates the problem described above in relation to the arrangement of FIG. 6 in which a voltage V1 is applied for a time period T3 upon switching the sub frame display.
Generally, at a spatial light modulating element, scanned pixel data are written one line at a time owing to restrictions on the amount of using such a spatial light modulating element that is based on a line-sequential system in the image display apparatus that is capable of multiplying the number of displayed pixels by a light path shifting unit, light shifting is conducted simultaneously on all the pixels of the spatial light modulating element, and thereby, a so-called image crosstalk may occur in which the same image may be displayed before and after the light path shifting resulting in the degradation of image quality.